1999 Chrysler 300M. My car would not start, so I jumped it and took it in for repair. Mechanic said my battery must have a loose plate and replaced the battery. Nothing changed. Mechanic reports battery voltage in car (engine not running) tests 12 volts sometimes, and tests 7.5 volts next time. Almost no resistance on cables. Suggestions?
The secret is where that 7.5 volts is being measured. Is it on the battery posts or on the cables that are attached to those posts? The secret is to start right on the posts, then move each voltmeter probe, one at a time, to the next place down the line to see where the voltage drops. This only works when current is being drawn from the battery. This test is normally done while trying to crank the engine, but in your case it might also work just by turning on the head lights.
Also, don't overlook the connections where the smaller red positive cable is bolted to the under-hood fuse box.